In order to reduce the weight of vehicles (cars or the like), it has been proposed that hollow springs (hollow coil springs or the like) be used as spring parts (for example, Patent Document 1). The hollow coil spring of Patent Document 1 is produced by performing a hot hydrostatic extrusion process, a thickness reduction process, a rolling process, a coil forming process, a heating process, an end surface polishing process, a shot peening process, a setting process, and a coating process in turn. In the hot hydrostatic extrusion process, a seamless spring steel pipe is formed. In the hollow coil spring, a spring material (a seamless spring steel pipe or the like) is used, so that fatigue strength to torsional stress and bending stress can be improved.
Patent Document 1 is Japanese Unexamined Patent Application Publication No. 2007-127227.